have been made to prop them up by cultural practices. These include artificial pollination, the use of disease-resistant and size controlling rootstocks, extensive methods of disease control, including complex schedules of pesticide application, the control of fruit size and annual bearing by manual and chemical fruit and flower thinning, the control of fruit abscission with growth regulators, and extensive pruning and training systems. Despite some intensive breeding programs, extremely successful in Prunus, strawberry, and blueberry, many of our fruit cultivars are ancient and based on grower-selected seedlings and somatic mutations (Table 2). Advances in molecular genetics may overcome some of the limitations to conventional fruit breeding based on sexual recombination by increasing selection efficiency using molecular markers and by transgene technology whereby individual genes from various sources may be inserted without disturbing unique genetic combinations. Progress has already been achieved in papaya (virus
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Avocado, Fruit Breeding, J. Janick, Persea